1. Field of the Invention
The present invention relates to circuits for communication devices. More particularly, the invention relates to a bridge circuit to suppress or damp echoes in communication devices.
2. Background Technology
In duplex mode, with a two-wire line, signals are transmitted simultaneously in both directions from and to a communication device, for example an xDSL modem. In order to be processed further, the incoming and outgoing signals must, however, be separated at both line ends into their respective transmission directions. Conversion therefore takes place from two-wire transmission to four-wire transmission and vice versa, performed by a hybrid circuit, for example.
FIG. 3 shows diagrammatically such an arrangement. Transmission and reception signals c are transmitted together on a two-wire line 4 from and to a remote end 5 of the line 4. At the near end of the line a hybrid circuit 6 performs the conversion from two-wire transmission to four-wire transmission. The transmission signal a is supplied to the hybrid circuit 6 at a connection 1 for a transmitter device, and the reception signal b is tapped or picked up at a connection 2 for a receiver device.
The problem arises here that the transmission signal a is reflected as an echo d into the reception signal b. Depending on the line type, the signal power of this echo can far exceed that of the reception signal. Hybrid circuits and/or filter solutions are designed to suppress this echo as far as possible.
A known possibility for echo suppression is the use of passive or active filters. This type of echo suppression has the advantage that the filters can be dimensioned independently of the transmission line. Depending on the transmission system used, these filters may, however, be very complex and hence cost-intensive. A further disadvantage of filter solutions is that the filters must be adapted accordingly for transmission and reception frequency bands that differ by system, which e.g., is possible only to a limited extent for integrated solutions.
Another solution approach is intended to simulate the echo path and cancel the echo with a signal obtained by simulation. This approach is generally known as Echo Cancellation. Examples of this are so-called balancing filters or 2nd DAC (digital-analog converter) solutions. The advantage of this method is the great z independence of the frequency bands used.
Both solutions can be designed to be relatively easily programmable, but are critical elements in the overall system in view of their linearity and noise contribution. The reason is that the echo suppression takes place first at the corresponding receiver or semiconductor module. After this suppression a great amplification is required to utilize the maximum signal level of the analog-digital converter (ADC) present in the reception signal path of such circuits.
Another possibility for echo cancellation is resistive or complex termination sets or bridge circuits. These circuits are dimensioned so that they simulate as well as possible a particular area of line types. Because of the various possible line properties it is necessary to make compromises in dimensioning and thus not obtain good echo suppression for every line type.
FIG. 4 shows the principle structure of a simple hybrid in the form of a bridge circuit. A combined transmission and reception signal c is transmitted via a transmitter 7 to the bridge circuit. The transmission signal a is supplied via a connection 1 to a transmission device, and the reception signal b is tapped via a connection 2 for a receiver device. An impedance Z1 can represent, for example, loads of a communication device comprising the bridge circuit, and an impedance Z2 can correspond to a winding in the transmitter 7 and serves to take into account the line impedance of the corresponding transmission distance or line. Impedances Z1 and Z2 are components of a first bridge branch of the bridge circuit. A second bridge branch formed from impedances Z3 and Z4 serves to simulate the path formed by Z1 and Z2 and thus balance the bridge circuit. The echo is minimized if equation 1
                              Z1          Z2                =                  Z3          Z4                                    (        1        )            is fulfilled. The transmission function of the transmission signal on the line is not influenced by the adjustment and is not considered further here.
In these circuits the problem arises that such a circuit cannot be implemented in an integrated circuit as the signal level in the second bridge branch is generally very high, although the signal power only amounts to a fraction of the transmission power as the second bridge branch is generally designed to have a high impedance.